1. Field of the Invention
The present invention is related to a thin film transistor having an active layer of an oxide semiconductor, a method for manufacturing the transistor and a film formation apparatus.
2. Description of the Related Art
In general, a thin film transistor using an amorphous silicon film, a polysilicon film or the like has been used for a transistor for driving an electric device. However, for forming a high quality amorphous silicon or polysilicon film it is necessary to have a temperature of 200 degrees Celsius or more during film formation. Therefore, it was difficult to obtain a flexible device which used an inexpensive polymer film as a supporting substrate.
In recent years, research and development into thin film transistors using an organic semiconductor material has been actively performed. As for an organic semiconductor material, a film can be formed by a printing process without using a vacuum process. Therefore, significant cost reductions may be possible. Further, there is a merit in which an organic semiconductor material can be formed on a supporting substrate such as a flexible plastic substrate. However, electron mobility of an organic semiconductor material is too low. Further, there is a demerit in which time degradation such as a poor quality resistance occurs in an organic semiconductor material. Therefore, an organic semiconductor has not yet been practically used in various fields.
From the above-mentioned reason, recent interest has focused on a transparent oxide semiconductor. As for a transparent oxide semiconductor, if it is manufactured at a low temperature, it has a high electron mobility. Besides, if a substrate, an electrode and an insulating film which are transparent are used, a transparent device is obtained. Further, the transparent oxide semiconductor is a material having new characteristics which conventional materials do not have. As a transparent oxide semiconductor, for example, a field-effect transistor using an amorphous In—Ga—Zn—O material has been proposed. Nomura et al. succeeded in forming a transparent field-effect transistor on a PET substrate at a room temperature, the transistor having a good property of mobility of about 10 cm2/Vs, by using an amorphous oxide semiconductor using the above-mentioned material as a semiconductor active layer. (See non-patent document 1.)
A thin film transistor using an oxide is manufactured by a vacuum process. Therefore, there is a problem in which costs of an apparatus are high compared with that of an organic semiconductor. Especially, in the case of a conventional batch type film formation apparatus, many vacuum film formation chambers are necessary. Therefore, costs of an apparatus are very high. Besides, there is a problem in which productivity is low.
In addition, in a field-effect transistor, controlling an interface between a semiconductor active layer and an insulating film is very important. It is known that characteristics of a transistor are lowered in the case where an interface between a semiconductor active layer and an insulating layer is contaminated.
Thus, the inventor tried to solve the problems such as controlling an interface of a conventional thin film transistor, low productivity and high apparatus costs. The present invention provides a method for manufacturing a bottom gate transistor having a gate electrode, a gate insulating film, an oxide semiconductor active layer, a source electrode and a drain electrode on a supporting material, including continuously forming a gate insulating film and an oxide semiconductor active layer without exposing it to air. According to the present invention, a clean interface between a gate insulating film and a semiconductor active layer without contamination is realized, and a thin film transistor having good characteristics can be provided.
A known document is described below.
[Non-patent document 1] K. Nomura et al. Nature, 432, 488 (2004)